The invention relates to a method for triggering a series spark gap, in which there are in series at least two partial spark gaps, and supply voltage is distributed over the partial spark gaps by means of first voltage distribution means.
The invention also relates to an arrangement for triggering a series spark gap, the series spark gap comprising at least two partial spark gaps in series, and the arrangement comprising first voltage distribution means for distributing supply voltage over the partial spark gaps.
For instance, in connection with high-voltage lines there are employed series capacitor batteries to compensate for line inductance. In parallel with the capacitor battery, in protection thereof, there is generally coupled a metal oxide varistor and/or a spark gap. The current-voltage characteristic of the metal oxide varistor is highly non-linear and as battery current rises, the metal oxide varistor limits capacitor voltage. A typical limiting voltage Ulim is 2.3 pu=2.3 Un, i.e. 2.3 times the nominal capacitor voltage (case-specifically the voltage may also be selected to be something else). This voltage passes over the capacitor with the maximum short-circuit current of the line. In a line short circuit the metal oxide varistor protects the capacitor by limiting its voltage to the value 2.3 pu. Thus, some of the current in the line passes through the metal oxide varistor that gets warm. In parallel with the capacitor and the metal oxide varistor there is coupled a so-called forced-triggered spark gap that is ignited if the varistor heats excessively. If the short circuit occurs in the same line sector where the series capacitor is located, forced-triggering of the spark gap is always attempted. Due to spark gap settings the typical lowest voltage at which the forced-triggering of the spark gap will succeed is about 2 pu using the conventional technology.
In a line short circuit line breakers switch the current off. If the line short circuit current is low, the varistor voltage does not always rise to the value 2 pu or higher. In that case the forced-triggering of the spark gap will not succeed. In case the capacitor battery has not been bypassed with a spark gap prior to opening the line breakers, a transient recovery voltage TRV of the line breakers rises. Therefore it is necessary for the forced-triggering of the spark gap to succeed with lower line current and capacitor voltage than 2 pu. A typical empirical requirement is about 1.7 to 1.8 pu.
SE publication 8 205 236 discloses an arrangement for forced triggering of a spark gap. The arrangement employs a separate pulse transformer that feeds a high-voltage pulse igniting the spark gap. By means of the high-voltage pulse there is ignited one of the auxiliary spark gaps arranged in parallel with the main spark gap, whereby these auxiliary spark gaps will be ignited eventually triggering the main spark gaps. It is necessary, however, to synchronize the ignition pulse with spark gap voltage so as to enable forced triggering. The synchronization and generation of energy needed by a high-voltage pulse and supply thereof to the pulse transformer require suitable means. These means make the structure of the forced-triggering device complicated, increase its costs and liability to damage and thus undermine the overall reliability of the forced-triggering device.
FI patent 80812 discloses an arrangement for forced-triggering a spark gap with voltage lower than autoignition. The spark gap is divided into at least two partial spark gaps in series. In parallel with the partial spark gaps there are coupled capacitors to provide mutual voltage distribution of the partial spark gaps. In series with the capacitors there is arranged a member controllably adopting a low impedance or high impedance state. On shifting to the high impedance state said member changes the mutual voltage distribution of the spark gaps such that the partial spark gap in parallel therewith ignites. The member adopting a high impedance or a low impedance state is a transformer, for instance. Strength of said member leaves a great deal to be desired. Moreover, the arrangement does not necessarily operate sufficiently fast.
There is further known an arrangement according to FIG. 1 for triggering a series spark gap. In the solution of FIG. 1 the main spark gap is divided into two partial spark gaps in series, i.e. a first partial spark gap 1 and a second partial spark gap 2. In parallel with the first partial spark gap 1 there are coupled capacitors Ca and Cb. In parallel with the second partial spark gap 2 there is coupled a capacitor Cc. The capacitors Ca, Cb and Cc are designed such that in a normal situation they distribute the voltage such that there is an equal voltage over both partial spark gaps 1 and 2. In parallel with the capacitor Cc there is coupled a first auxiliary spark gap 3. In series with the first auxiliary spark gap 3 there is coupled a first current limiting resistor R1. In parallel with the capacitor Cb there is coupled a second auxiliary spark gap 4 and in series therewith there is coupled a second current limiting resistor R2. The auxiliary spark gaps 3 and 4 are gas-pressure spark gaps, i.e. trigatrons. They are hermetically closed, and therefore their autoignition voltage is constant, in principle. There is, however, a slight spread in their ignition voltage, and thus, to be on the safe side, their autoignition voltage is set to a value that is about 10% higher than the highest voltage over them, which is 2.3 pu/4=0.575. In said example the setting is thus 1.1*2.3/4=0.633 pu. When the series spark gap is to be triggered, the procedure is as follows. A trigger pulse is fed to the first auxiliary spark gap. This provokes ignition in the first spark gap, and consequently the capacitor Ca is discharged through the current limiting resistor R1. The voltage is then distributed such that one third of the voltage acting over the whole arrangement acts over the capacitor Cb and thus over the second auxiliary spark gap 4.
Autoignition voltage of the second auxiliary spark gap is set to value 1.1*2.3/4=0.633 pu. This voltage passes over said second auxiliary spark gap if the voltage acting over the whole spark gap is 3*0.633 pu=1.9 pu. In view of the tolerance of the auxiliary spark gap the required voltage over the whole spark gap is 2 pu.
In series with the current limiting resistor R1 there is a transformer 5 that gives a trigger pulse to the second auxiliary spark gap 4. The trigger pulse expedites ignition, but does not necessarily decrease the ignition voltage, because the trigger pulse has very short duration. When the second auxiliary spark gap 4 ignites, the capacitor Cb discharges through the resistance R2. This results in the whole voltage acting over the second partial spark gap that will ignite. Thereafter the first partial spark gap will also ignite.
Autoignition of the auxiliary spark gaps 3 and 4 may not be set excessively low so that they would not ignite on their own without forced triggering. As described above, the whole spark gap will be ignited at voltage 2.0 pu, if the limiting voltage of the varistor is 2.3 pu. In all cases the value 2.0 pu is not sufficiently low, however. The arrangement is also relatively complicated and consequently expensive.